Oxidation of a refractory high entropy alloy (RHEA) at moderate temperatures for wear related applications

This study was initiated to exploit the poor oxidation resistance of a RHEA, namely HfNbTaTiZr, to enhance its surface hardness and wear performance. In this regard, samples sliced from vacuum arc melted HfNbTaTiZr ingots having a hardness of ∼400 HV were held at two different temperatures (450 and 600 °C) for 3 h in air. The structural examinations conducted by using X-ray diffraction (XRD) and Raman analysis along with an energy dispersive spectrometer (EDS) equipped scanning electron microscope (SEM) revealed that the samples were covered with a complex HfNbTaTiZrO11 type oxide layer (OL) having the hardness of ∼1550 HV. The average surface roughness (Ra) and the thickness of the OLs were measured as ∼0.13 μm and ∼ 1 μm after oxidation at 450 °C and ∼ 0.29 μm and ∼ 8 μm after oxidation at 600 °C, respectively. The protective nature of the OLs was confirmed by dry sliding wear tests employed at room temperature against alumina balls under a contact pressure of ∼0.5 GPa. This led to 240- and 45-times higher wear resistance for the samples oxidized at 450 and 600 °C, respectively. During the wear tests, the untreated sample was exposed to severe wear owing to the accumulation of heavy plastic deformation at the contact surface, while for the oxidized samples wear progressed in an elastic deformation regime on the OLs resulting in mild wear. However, the roughness of the OLs played a crucial role on the wear resistance of the oxidized samples, so that the oxidation temperature of 600 °C, which imposed a higher R_a value, resulted in a reduction in wear resistance when compared to the oxidation temperature of 450 °C, which formed a smoother OL.